(1) Field of the Invention
The invention relates to a process for the manufacture of a highly photosensitive amorphous silicon thin film phototransistor and the resulting phototransistor.
(2) Description of Related Prior Art
The various known techniques for making large area thin film photodetectors include the use of amorphous silicon semiconductor, because the deposition temperature is low enough that it can be deposited on a glass substrate. A glass substrate is preferred since it allows the formation of a large area photodetector. In almost all cases up to the present time, the resulting device structure is either a photodiode or a photoconductor.
The photodiode exhibits low dark current, because the diode is a reverse biased PN junction or is inserted between the amorphous silicon layer and the electrode. In the photodiode the photocurrent is a primary current that saturates with unity collection efficiency. This, of course gives an optical gain of less than one.
In the case of the photoconductor sensor, the electrode is ohmic where carrier replenishment occurs, electron photoconductivity is the dominant mode. The photocurrent is a secondary current. Photocurrent and photoconductive gain is proportional to the mobility lifetime product of amorphous silicon. This gives a photoconductive gain of about ten. To increase this gain, a linear sensor with interdigital geometry structure may be added to produce a gain as high as much as 1000. However, the dark current of the photoconductor, which increases proportional to the applied voltage is much larger than the photodiode.
Recently an amorphous silicon photo transistor has been suggested in the article "Amorphous Silicon Phototransistor on a Glass Substrate" by B. S. Wu et al and publishes in IEEE Transactions on Electron Devices, Vol. ED-32, No. 11, November 1985 pages 49-53. In this article an amorphous silicon N+-IP+-IN+ thin film phototransistor on a glass substrate is described. However, in order to increase optical gain of the phototransistor, a very thin base layer must be used in the phototransistor. Therefore, the emitter-base potential is not high enough to block the electron flux which is emitted from the emitter to the collector. The dark current is still very high in this phototransistor.